Fault current limiting

ABSTRACT

An alternating current system  10  has a primary circuit  11  which forms a primary winding  18  on a core  16.  A secondary winding  24  is connected with a current source  26  or, alternatively, with an impedance  60.  The core  16  is threaded by a superconducting coil  20  having a current source  22.    
     In normal use, current in the coil  20  provides a DC bias level of flux in the core  16,  and the source  26  is varied to maintain substantially constant flux, thereby minimising losses in the primary circuit  11.  In fault conditions, current in the coil  20  is reduced or removed to increase voltage losses across the coil  18,  thereby limiting fault current. The impedance  60  can also be switched into circuit, creating further current limiting by virtue of the transformer effect of the windings  18, 24.

The present invention relates to fault current limiting. In particular,but not exclusively, the invention relates to limitation of faultcurrents within electrical distribution systems.

Fault conditions within electrical distribution systems have thepotential for creating high fault current levels. Electricaldistribution systems must either be designed to be capable of handlinghigh fault current levels, or must employ current limiting devices toreduce or limit fault currents and their consequences, until the faultis isolated, for example by switchgear.

In one aspect, the present invention provides a fault current limitingarrangement, comprising:

a primary circuit for current to be limited in the event of a fault,

a core having a primary winding forming part of the primary circuit;

a coil coupled with the core, and a DC current supply for the coil, toprovide a bias level of flux in the core;

an auxiliary winding coupled with the core;

and an auxiliary current source for the auxiliary winding, the output ofthe auxiliary current source being adjustable in response to changes atthe primary winding to apply control to the primary winding by controlof the flux within the primary winding.

Preferably, the coil comprises a superconducting element. The coilcurrent supply is preferably operable, in use, to set the coil currentto saturate the core. The coil current supply is preferably operable, inuse, to change the coil current in response to a fault conditionoccurring in the primary circuit. The coil current supply is preferablyoperable to remove the coil current in response to a fault condition.

The auxiliary current source may be operable to control the auxiliarycurrent to maintain substantially constant flux within the primarywinding, during use, except during fault conditions. The primary circuitis preferably connected with one phase of a multi-phase electricalsystem, the auxiliary current source being operable to control theauxiliary current to change the primary winding flux, thereby enabling acontrollable inductance that can provide VAR compensation. The auxiliarycircuit may include impedance for switching into circuit during faultconditions.

The core preferably has a relatively thin arm on which the primarywinding is wound, and a relatively thick arm on which the auxiliarywinding is wound. In use, the bias level of flux provided by the coil ispreferably sufficient to saturate the relatively thin arm and isinsufficient to saturate the relatively thick arm in the presence ofcurrent in the primary or auxiliary windings.

In another aspect, the present invention provides a method of faultcurrent limiting in which:

a core is provided having a primary winding forming part of a primarycircuit;

a coil is coupled with the core, and provided with DC current to providea bias level of flux in the core;

an auxiliary winding is coupled with the core;

and current is provided for the auxiliary winding and adjusted inresponse to changes at the primary winding to apply control to theprimary winding by control of the flux within the primary winding.

Preferably, the coil comprises a superconducting element. The coilcurrent supply preferably sets the coil current to saturate the core.The coil current preferably changes in response to a fault conditionoccurring in the primary circuit. The coil current is preferably removedin response to a fault condition.

The auxiliary winding current is preferably controlled to maintainsubstantially constant flux within the primary winding, during use,except during fault conditions. If the primary circuit is connected withone phase of a multi-phase electrical system, the auxiliary windingcurrent may be used to control the auxiliary current to change theprimary winding flux, thereby to provide VAR compensation.

In another aspect, the present invention provides a fault currentlimiting arrangement, comprising:

a primary circuit for current to be limited in the event of a fault,

a core having a primary winding forming part of the primary circuit;

a coil coupled with the core, and a DC current supply for the coil, toprovide a bias level of flux in the core; wherein the coil currentsupply is operable, in use, to change the coil current in response to afault condition occurring in the primary circuit.

The coil current supply is preferably operable to remove the coilcurrent in response to a fault condition. Preferably, the coil comprisesa superconducting element. The coil current supply is preferablyoperable, in use, to set the coil current to saturate the core.

The auxiliary current source may be operable to control the auxiliarycurrent to maintain substantially constant flux within the primarywinding, during use, except during fault conditions. The primary circuitis preferably connected with one phase of a multi-phase electricalsystem, the auxiliary current source being operable to control theauxiliary current to change the primary winding flux, thereby to provideVAR compensation. The auxiliary circuit may include impedance forswitching into circuit during fault conditions.

The core preferably has a relatively thin arm on which the primarywinding is wound, and a relatively thick arm on which the auxiliarywinding is wound. In use, the bias level of flux provided by the coil ispreferably sufficient to saturate the relatively thin arm and isinsufficient to saturate the relatively thick arm in the presence ofcurrent in the primary or auxiliary windings.

In another aspect, the present invention provides a method of faultcurrent limiting, in which:

a core is provided with a primary winding forming part of a primarycircuit for current to be limited in the event of a fault,

a coil is coupled with the core, and provided with a DC current toprovide a bias level of flux in the core; and

the coil current is changed in response to a fault condition occurringin the primary circuit.

The coil current is preferably removed in response to a fault condition.Preferably, the coil comprises a superconducting element. The coilcurrent supply is preferably set to saturate the core.

The auxiliary winding current is preferably controlled to maintainsubstantially constant flux within the primary winding, during use,except during fault conditions. If the primary circuit is connected withone phase of a multi-phase electrical system, the auxiliary windingcurrent may be used to control the auxiliary current to change theprimary winding flux, thereby to provide VAR compensation.

Examples of the present invention will now be described in more detail,by way of example only, and with reference to the accompanying drawings,in which:

FIG. 1 is a schematic diagram of an electrical system with fault currentlimiting arrangements in accordance with the present invention;

FIG. 2 is a simplified diagram of the core and winding topology of thesystem of FIG. 1;

FIG. 3 is a plot of flux against current in the core of FIG. 2 in theabsence of the auxiliary winding; and

FIG. 4 is a schematic diagram, corresponding with FIG. 1, illustrating athree phase system.

FIG. 1 is a schematic diagram of an alternating current system 10 havinga conductor 11 between terminals 12. Alternating voltages and currentsV_(a) and i_(a) arise, in use, within the system 10. The system 10 maybe, for example, an electrical distribution system.

The system 10 has a fault current limiting arrangement 14. The conductor11, between the terminals 12, forms a primary circuit within thearrangement 14, being the circuit within which current is to be limitedin the event of a fault. That is to say, when the electricaldistribution system represented by the conductor 11 experiences a fault,the resulting fault currents in the conductor 11 are to be limited bythe arrangement 14, until other control arrangements, such as isolators,can come into circuit to isolate the fault.

The arrangement 14 has a core 16 of iron or other magnetic material,illustrated schematically by parallel lines in FIG. 1. The conductor 11forms a primary winding 18 on the core 16.

A coil 20 threads the core 16 and is therefore coupled with the core. ADC current supply 22 for the coil 20 causes the coil 20 to provide abias level of flux in the core 16.

An auxiliary winding 24 is coupled with the core 16 and has an auxiliarycurrent source 26. The output of the current source 26 is adjustable inresponse to changes at the primary winding 18, detected at 28 by currentthrough the conductor 11, the voltage across the winding 18 or flux inthe core 16. The output of the source 26 is adjusted, in use, inresponse to changes at the primary winding 18, to apply control to theprimary winding 18 by control of the flux within the primary winding 18,as will be described.

Operation of the arrangement 14 can best be described by firstdescribing the topology of the core 16 in more detail. This isillustrated in FIG. 2.

In the example shown in FIG. 2, the core 16 forms a closed loop of iron,having a relatively thin arm 30 on which the primary winding 18 iswound, and a relatively thick arm 32 on which the auxiliary winding 24is wound. The arms 30, 32 are connected by legs 34, so that the arms 30,32 and legs 34 form a continuous loop for magnetic flux within the core16. Accordingly, the primary winding 18 and the auxiliary winding 24 arecoupled by the core 16 in the manner of a transformer.

The core 16 is also threaded by the coil 20. The coil 20 is preferably asuperconducting coil. Accordingly, current created in the coil 20, bythe supply 22, will be maintained with high efficiency, while the coil20 remains superconducting. In order to maintain conditions for the coil20 to remain superconducting without quenching, arrangements areprovided in the form of a cryogenic tube 36 around the coil 20, so thatthe core 16 can be operated at ambient temperatures, while the coil 20operates at cryogenic temperatures. Allowing the components other thanthe coil 20 to operate at ambient temperatures results in higherresistivity in those elements than would occur at cryogenictemperatures, with advantages which will become apparent from thefollowing description.

FIG. 3 illustrates the relationship between current and magnetic fluxwithin the arrangement of FIG. 2, in the absence of the auxiliarywinding 24, during normal use and also in fault conditions.

FIG. 3 plots, on the vertical axis, magnetic flux within the core 16,against (horizontal axis) the product of the number of turns of theprimary winding 18 and the current in the primary winding 18. Thischaracteristic 38 has a shape which is initially substantially linear at40, reaching saturation at 42, so that flux increases much more slowlywith increasing winding current, after saturation 42.

The effect of current in the coil 20, which is also coupled with thecore 16, is to induce magnetic flux in the core 16. The current in thecoil 20 is DC, so that the flux in the core 16 operates at a DC bias setby the current in the coil 20. The DC bias is sufficient to saturate thecore 16. That is, the bias flux is beyond the saturation point 42. Thisis illustrated at 44. Accordingly, the effect of the current in theprimary winding 18 on the flux in the core leg 30, during non-faultconditions, is equivalent to superimposing the normal current i_(a) 46on a DC bias current represented by 44. Since the core leg 30 issaturated, the current 46 results in only a small fluctuation 48 in theinduced magnetic flux in the core 16. This leads to a small voltage dropacross the arrangement 14 within the system 10, because the devicevoltage for an inductive arrangement such as the primary winding 18 isproportional to the rate of change of magnetic flux enclosed within thewinding 18. Accordingly, in the absence of further arrangements to bedescribed, the small fluctuations 48 would result in a power loss withinthe arrangement 14. In particular, some of this power loss would imposea load on the cryogenic arrangements 36. The losses may also affectvoltage regulation within the primary circuit.

The auxiliary winding 24, in conjunction with the auxiliary currentsource 26 and detector 28 are used to provide compensation for thisvoltage loss, in the following manner. The auxiliary current source 26is set to operate in response to changes at the primary winding 18,detected by the detector 28, in order to compensate for those changesand provide a feedback arrangement which maintains a substantiallyconstant magnetic flux within the core 16, compensating for therelatively small changes in flux induced by the changes of currentwithin the primary winding 18.

This indicates the significance of the different sizes of the arms 30,32. The thin arm 30 can be maintained beyond saturation 42 without thethick arm 32 being saturated and this, in turn, allows the auxiliarywinding 24 to be used to change the flux in the arm 32, to maintain thethin arm 30 in a saturated state.

Accordingly, during non-fault conditions, the operation of the auxiliarywinding 24 seeks to maintain the flux in the core 16 at the bias levelcorresponding with the DC bias 44, substantially at all times. Thisreduces, minimises or eliminates the voltage drop across the arrangement14, in non-fault conditions.

When fault conditions arise, high fault currents can be expected toarise, as noted above and as illustrated at 50 (FIG. 3). The magnitudeof the AC fault current 50 is much greater than the magnitude of thecurrent i_(a) in non-fault conditions. However, since the DC bias 44 isbeyond the saturation point 42, one half cycle 50 a of the fault current50 results in little additional flux 51 a in the core 16 and thusresults in relatively little voltage drop across the arrangement 14, andthus relatively little limitation of the fault current 50. However, theother half cycle 50 b of the fault current 50 takes the core 16 belowthe saturation point 42, resulting in a much greater change 51 b in theflux, to reduce the flux significantly and thus give rise to asignificant voltage drop 52 across the device. This causes limitation ofthe fault current, but only during the half cycle 50 b.

In the example being described, this ability to limit fault current isimproved by incorporating a second detector 54, illustratedschematically in FIG. 1, to detect fault conditions in the system 10 andto control the DC current supply 22 so that, in the event of faultconditions arising, the DC current supply 22 is switched so that thecurrent in the coil 20 is no longer maintained. The supply 22 may beopen circuited, but preferably the supply 22 is able to dissipate thecurrent in the coil 20, thereby reducing the coil current towards zero.This rapidly removes the DC bias 44, resulting in the core 16 operatingabout the origin 56 of the characteristic 38, so that both half cyclesof the fault current give rise to significant flux within the core 16and therefore significant voltage drops across the arrangement 14. Thisconverts the arrangement 14, during fault conditions, to an inductivefault current limiter.

In addition, rapid removal of the DC bias current in the coil 20 alsoprotects the superconducting coil 20 from being quenched to anon-superconducting state by high currents induced in the coil 20 byfault currents in the primary winding 18, or by high magnetic fieldsarising in a similar manner.

A further current limiting effect is available within the arrangement14, during fault conditions detected by the detector 54. In thisexample, the auxiliary current source 26 is connected with the auxiliarywinding 24 through switches 58, which can be switched to disconnect theauxiliary current source 26 from the winding 24, simultaneouslyconnecting a resistance 60 in series with the auxiliary winding 24. Thisis done in response to the onset of fault conditions, when the auxiliarycurrent source 26 is no longer required to maintain the core flux at theDC bias level 44, as has been described.

When the resistance 60 is in circuit with the auxiliary winding 24, thetransformer provided by the core 16, primary winding 18 and auxiliarywinding 24 results in the resistance 60 appearing as a series resistancein the conductor 11, having a value (N_(p) ²/N_(a) ²)R, where N_(p) isthe number of turns of the primary winding 18, N_(a) is the number ofturns of the auxiliary winding 24 and R is the value of the resistance60. Thus, the resistance 60 also provides resistive fault currentlimitation in the conductor 11.

FIG. 4 illustrates a further example system 10 a having many featuresequivalent with features described above in relation to FIG. 1.Accordingly, corresponding figures are given the same reference numeralsin FIG. 5 as in FIG. 1, with a letter suffix a, b etc. The system 10 ais a three phase alternating current system of conductors 11 a, b, c ofthe three phases, between terminals 12 a, 12 b, 12 c. Each phase of thesystem 10 a has a fault current limiting arrangement 14 a, including acore 16 a, primary winding 18 a, coil 20 a, DC current supply 22 a,auxiliary winding 24 a, auxiliary current source 26 a, and detector 28a. In addition, each phase has switches 58 a and resistance 60 a.Accordingly, each phase has the structure to allow operation in themanner described above.

In addition, a further control arrangement 70 is provided. The controlarrangement 70 is common to the three phases, receives inputs from thethree detectors 28 a and provides control signals to the three auxiliarycurrent sources 26 a. The control arrangement 70 operates to control theauxiliary current sources 26 a for the purposes described above,particularly to adjust the auxiliary current sources 26 a in response tochanges at the primary winding 18 a of the corresponding phase, to applycontrol to the primary winding 18 a by control of the flux within theprimary winding 18 a. Thus, each phase of the system 10 a can becontrolled in the manner described above. In addition, the controlarrangement 70 is operable to provide control of reactive power withinthe three phase system 10 a. Control of reactive power is commonlyreferred to as VAR control or VAR compensation. In the system 10 a, thedetectors 28 a are used to measure voltage and current at the primarywinding 18 a of the corresponding phase, so that the control arrangement70 has information about all three phases. Consequently, the controlarrangement 70 is able to adjust the output of any of the auxiliarycurrent sources 26 a, independently of the sources 26 a of the otherphases, in order to change the DC bias position at which the phase isoperating. This allows a change of magnetic flux to be created withinthe core of the corresponding phase, thus creating a change of voltageacross the primary winding of the corresponding phase. This in turngives rise to VAR compensation between the three phases.

Many variations and modifications can be made to the examples describedabove, without departing from the scope of the present invention. Forexample, the coils 20, 20 a have been described as superconducting.Superconducting coils provide efficient supply of a DC bias current forthe reasons described, but non-superconducting arrangements couldalternatively be used. The ability of the examples to control the DCbias level, to remove the DC bias level in fault conditions, to achieveVAR compensation between phases and to create a resistive fault currentlimiter by switching a resistance into circuit with the auxiliarywinding can each be used alone or in various combinations with the otherfeatures.

1. A fault current limiting arrangement, comprising: a primary circuitfor current to be limited in the event of a fault, a core having aprimary winding forming part of the primary circuit; a coil coupled withthe core, and a DC current supply for the coil, to provide a bias levelof flux in the core; an auxiliary winding coupled with the core; and anauxiliary current source for the auxiliary winding, means to adjust theoutput of the auxiliary current source in response to changes at theprimary winding to apply control to the primary winding by control ofthe flux within the primary winding, wherein the auxiliary currentsource is operable to control the auxiliary current to maintainsubstantially constant flux within the primary winding, during use,except during fault conditions.
 2. An arrangement according to claim 1wherein the coil comprises a superconducting element.
 3. An arrangementaccording to claim 1 wherein the coil DC current supply is operable, inuse, to set the coil current to saturate the core.
 4. An arrangementaccording to claim 1 wherein the coil DC current supply is operable, inuse, to change the coil current in response to a fault conditionoccurring in the primary circuit.
 5. An arrangement according to claim 4wherein the coil DC current supply is operable to remove the coilcurrent in response to a fault condition.
 6. An arrangement according toclaim 1, wherein the primary circuit is connected with one phase of amulti-phase electrical system, the auxiliary current source beingoperable to control the auxiliary current to change the primary windingflux, thereby to provide VAR compensation.
 7. An arrangement accordingto claim 1, wherein the auxiliary winding forms an auxiliary circuit,the auxiliary circuit includes an impedance and switches for switchingthe impedance into the auxiliary circuit during fault conditions.
 8. Anarrangement according to claim 1, wherein the core has a relatively thinarm on which the primary winding is wound, and a relatively thick arm onwhich the auxiliary winding is wound.
 9. An arrangement according toclaim 8, wherein in use, the bias level of flux provided by the coil issufficient to saturate the relatively thin arm and is insufficient tosaturate the relatively thick arm in the presence of current in theprimary or auxiliary windings.
 10. A method of fault current limiting inwhich: a core is provided having a primary winding forming part of aprimary circuit; a coil is coupled with the core, and provided with DCcurrent to provide a bias level of flux in the core; an auxiliarywinding is coupled with the core; and current is provided for theauxiliary winding and adjusted in response to changes at the primarywinding to apply control to the primary winding by control of the fluxwithin the primary winding, wherein the auxiliary winding current iscontrolled to maintain substantially constant flux within the primarywinding, during use, except during fault conditions.
 11. A methodaccording to claim 10, wherein the coil comprises a superconductingelement.
 12. A method according to claim 10, wherein the coil currentsupply sets the coil current to saturate the core.
 13. A methodaccording to claim 10, wherein the coil current changes in response to afault condition occurring in the primary circuit.
 14. A method accordingto claim 10, wherein the coil current is removed in response to a faultcondition.
 15. A method according to claim 10, wherein the primarycircuit is connected with one phase of a multi-phase electrical system,and the auxiliary winding current is used to control the auxiliarycurrent to change the primary winding flux, thereby to provide VARcompensation.
 16. A fault current limiting arrangement, comprising: aprimary circuit for current to be limited in the event of a fault, acore having a primary winding forming part of the primary circuit; acoil coupled with the core, and a DC current supply for the coil, toprovide a bias level of flux in the core; wherein the coil currentsupply is operable, in use, to change the coil current in response to afault condition occurring in the primary circuit.
 17. An arrangementaccording to claim 16, wherein the coil current supply is operable toremove the coil current in response to a fault condition.
 18. Anarrangement according to claim 16, wherein the coil comprises asuperconducting element.
 19. An arrangement according to claim 16,wherein the coil current supply is operable, in use, to set the coilcurrent to saturate the core.
 20. An arrangement according to claim 16,wherein an auxiliary current source is operable to control an auxiliarycurrent to an auxiliary winding coupled with the core to maintainsubstantially constant flux within the primary winding, during use,except during fault conditions.
 21. An arrangement according to claim20, wherein the primary circuit is connected with one phase of amulti-phase electrical system, the auxiliary current source beingoperable to control the auxiliary current to change the primary windingflux, thereby to provide VAR compensation.
 22. An arrangement accordingto claim 20, wherein the auxiliary winding forms an auxiliary circuit,the auxiliary circuit includes an impedance and switches for switchingthe impedance into the auxiliary circuit during fault conditions.
 23. Anarrangement according to claim 20, wherein the core has a relativelythin arm on which the primary winding is wound, and a relatively thickarm on which the auxiliary winding is wound.
 24. An arrangementaccording to claim 23, wherein in use, the bias level of flux providedby the coil is sufficient to saturate the relatively thin arm and isinsufficient to saturate the relatively thick arm in the presence ofcurrent in the primary or auxiliary windings.
 25. A method of faultcurrent limiting, in which: a core is provided with a primary windingforming part of a primary circuit for current to be limited in the eventof a fault, a coil is coupled with the core, and provided with a DCcurrent to provide a bias level of flux in the core; and the coilcurrent is changed in response to a fault condition occurring in theprimary circuit.
 26. A method according to claim 25, wherein the coilcurrent is removed in response to a fault condition.
 27. A methodaccording to claim 25, wherein the coil comprises a superconductingelement.
 28. A method according to claim 25, wherein the coil currentsupply is set to saturate the core.
 29. A method according to claim 25,wherein an auxiliary winding current in an auxiliary winding coupledwith the core is controlled to maintain substantially constant fluxwithin the primary winding, during use, except during fault conditions.30. A method according to claim 25, wherein the primary circuit isconnected with one phase of a multi-phase electrical system, and anauxiliary winding current is used to control an auxiliary current to anauxiliary winding coupled with the core to change the primary windingflux, thereby to provide VAR compensation.